OUTSIDE THE SQUARE
Over its 27 years of production, the Square Four evolved through progressive advancements attaining limitless reserves of pulling power and innate smoothness. The model was a longstanding and highly successful design for Ariel. The SQ4 has also been openly referenced by Japanese marquees as a key influence in their early pursuit of the perfect four-cylinder machine.

Edward Turner conceived the Square Four engine in 1928. At this time he was looking for work, showing drawings of his engine design to motorcycle manufacturers. The engine was essentially a pair of ‘across frame’ OHC parallel twins joined by their geared central flywheels, with one four-cylinder block (or Monobloc) and one head. The idea for the engine was rejected by BSA, but adopted by Ariel, thus it became the ‘Ariel Square Four’.

Turner’s vision was to provide “a four-cylinder engine small enough for use in solo motorcycles yet producing ample power for high performance without undue compression, racing cams or big choke carburettors”. Essentially, Edward Turner’s design philosophy foreshadowed decades of modern design. He stated that he merely sought to “deliver ultimate reliability and performance with minimum attentions”.

In 1949, the Ariel Square Four Mark One saw the cast-iron head and barrel replaced by a modern alloy head and barrels. Casting off 20 kg in weight and further improving maintenance requirements, the Square Four refreshed itself to the public on the eve of two-stroke madness and the ensuing arrival of small-capacity multi cylinders. Delivering 90mph, the SQ4 in many ways was, and remains, the forefather of the sports cruiser.